Rotary valve internal combustion engines having a rotary valve that rotates within a bore in the engine's cylinder head with a predetermined clearance have been described in several patents, including U.S. Pat. No. 5,526,780 (Wallis) and U.S. Pat. No. 4,852,532 (Bishop). Such rotary valve arrangements must have a sealing mechanism to seal the gap between the cylinder head bore and the rotary valve. This is preferably achieved by an array of floating gas sealing elements surrounding a window in the bore that communicates with the combustion chamber. Such sealing systems are also described in U.S. Pat. No. 5,526,780 (Wallis) and U.S. Pat. No. 4,852,532 (Bishop). Typically, each seal element is located in a corresponding slot in the cylinder head bore and preloaded against the outer diameter of the rotary valve. These sealing elements must seal the full cylinder combustion pressure, which may peak at 100 bar or greater. Each sealing element is designed to operate in a similar manner to a piston ring. The combustion gas from the cylinder flows into the slot, pushing the seal element against the outer face of the slot, and flows under the seal element to push it against the outside diameter of the valve. In this way the gas pressure acts to press the seal element against the two surfaces that it must seal with a force that is proportional to the pressure it must seal.
FIG. 1 shows a prior art axial sealing element, similar to that disclosed in U.S. Pat. No. 5,526,780 (Wallis), assembled into a rotary valve engine with a schematic steady state pressure distribution shown around it. In this example, prior art sealing element 31 is an axial sealing element with a constant rectangular cross section and parallel sides. Sealing element 31 is located in an axial slot 20 in the cylinder head bore 8, parallel to the axis of the rotary valve. Sealing element 31 is biased against the cylindrical portion 4 of the outside of the rotary valve by springs not shown. The force applied by these springs is indicated by arrow F. The clearances shown are exaggerated for the purposes of explaining the operation of the sealing element, and in practice the clearances are minimal. The pressure in the combustion chamber is indicated by Pc and the flow of combustion gases into slot 20, through the clearance between the side of sealing element 31 and the side 35 of slot 20 closest to the combustion chamber window, as combustion pressure Pc increases, is indicated by flow arrow 32. The pressure Pus on the underside 34 of sealing element 31, which is the same as the pressure in the bottom of slot 20, is equal to the combustion pressure Pc. In this state, sealing element 31 seals the gap between cylinder head bore 8 and cylindrical portion 4 by being pressurised against cylindrical portion 4 and the side 33 of slot 20 furthest from the combustion chamber window.
The surfaces of the sealing element, the slot and the outside diameter of the valve are not perfectly flat and smooth so there will be a small amount of leakage across the faces that are to be sealed. This allows the establishment of a pressure distribution on these surfaces opposing the closing force generated by the free access of gas to the surfaces opposite to the sealing surfaces. This pressure distribution is typically triangular in shape as is shown in FIG. 1 by the pressure distribution between sealing element 31 and cylindrical portion 4, and between sealing element 31 and the side 33 of slot 20 furthest from the combustion chamber window.
Generally speaking, at slow to moderate engine speeds, Pus is approximately equal to Pc (as shown in FIG. 1) with the result that the sealing elements are adequately pressurised against the valve to seal successfully. However, at high engine speeds the pressure Pus on the underside of the sealing element may be inadequate to resist the combustion pressure and as a result, the sealing element may be forced away from the valve causing the sealing to fail.
The volume underneath the sealing element determines the mass of gas that must be transported into this area in order to pressurise it. This volume acts as a capacitor and consequently the pressure Pus on the underside of the sealing element lags the combustion pressure Pc that the sealing element must seal against. The magnitude of this pressure lag is a function of the volume, the flow area feeding the volume, the radial depth of the sealing element, and the engine speed. The pressure lag increases when engine speed, volume or radial depth increases, or flow area decreases. The flow area is proportional to the side clearance of the sealing element in its slot, which is generally kept to an absolute minimum consistent with the sealing element always being free to move. This minimises the crevice volume and the fore and aft movement of the sealing element in its slot. The gas velocity through this side clearance is proportional to the pressure lag but the velocity is limited to Mach 1, which is when the flow becomes choked.
Minimising the clearance between the underside of the sealing element and the bottom of its slot minimises the volume underneath the sealing element. However, the sealing element must have some clearance for proper operation and assembly, and therefore there is a limit to the extent that this volume can be minimised.
FIG. 2 is the same as FIG. 1 except with a pressure lag as described above. The pressure lag is graphically indicated by the magnitude of Pus being significantly less than Pc. It can be easily demonstrated that in order for sealing element 31 to remain preloaded against cylindrical portion 4 the following condition must be true.Pus>0.5(Pc)−F/A 
Where ‘A’ is the area of the underside 34 of sealing element 31 and so ‘F/A’ is the effective pressure generated by bias spring force ‘F’. However, ‘F/A’ is generally small compared to the combustion pressure Pc, so the above condition can be simplified as follows.Pus>0.5(Pc)
For any given sealing element arrangement, the pressure lag will increase with increasing engine speed until a point is reached where this condition is no longer true and the force acting on the top face of the sealing element exceeds that acting on its underside. When this occurs the sealing element will be forced away from the surface of the rotary valve with consequent collapse of sealing. The engine speed at which this occurs will be a function of the sealing element side clearance, radial depth and the volume underneath the sealing element.
In some circumstances, a sealing element may be forced into contact with the side of its slot that is closest to the combustion chamber window. A typical example of this is the leading axial sealing element. The friction between the sealing element and the rotating valve, against which it is preloaded, pushes the leading axial sealing element into contact with the slot side closest to the window. Another situation where this occurs is when the engine is running at closed throttle and a high vacuum exists in the cylinder, which tends to pull the sealing elements inwards against the sides of their slots closest to the window. In these situations the rising cylinder pressure during compression and combustion must first push the sealing element away from its slot side closest to the window, before gas flow can commence between the sealing element and the slot side to the underside of the sealing element. However, in these circumstances the cylinder pressure initially only sees that portion of the sealing element protruding above the slot. Consequently considerable pressure is required to force the sealing element away from the slot side closest to the window, which exacerbates the pressure lag and lowers the engine speed at which collapse of sealing commences.
The present invention seeks to provide a sealing element that at least ameliorates some of the problems of the prior art.